1. Field of the Invention
The present invention relates to ultraviolet sensors, including diamond films, for detecting ultraviolet light without being affected by solar light and also relates to method for manufacturing such sensors. The present invention particularly relates to an ultraviolet sensor including a highly oriented diamond film and a method for manufacturing such a sensor.
2. Description of the Related Art
In recent years, various ultraviolet sensors have been proposed (see, for example, Japanese Unexamined Patent Application Publication Nos. 5-335613 and 11-248531; UV Photodetectors from Thin Film Diamond; Phys. Stat. Sol. (a), 1996, vol. 154, pp. 445–454; Diamond UV photodetectors: sensitivity and speed for visible blind applications; Diamond and Related Materials, 1998, vol. 7, pp. 513–518; Polycrystalline diamond photoconductive device with high UV-visible discrimination; and Appl. Phys. Lett., 1995, vol. 67, pp. 2117–2119). Such diamond films are useful in manufacturing highly reliable sensors with low cost because they have high resistivity to high temperature, durability, and nature of semiconductor elements having a wide band gap. Semiconductor sensors including the diamond films are superior in durability as compared to known sensor devices including wavelength filters and silicon films or the like with a narrow band gap. Furthermore, there is an advantage in that the semiconductor sensors need not include complicated circuits and are smaller in size and weight as compared to known sensors including phototubes or the like.
In order to reduce manufacturing cost, the ultraviolet sensors disclosed in the above documents usually include polycrystalline diamond films formed by a vapor deposition process and each include, for example, a pair of electrodes arranged thereon. FIG. 5 is a schematic sectional view showing a known ultraviolet sensor 100 with a coplanar structure. The known ultraviolet sensor 100 includes, for example, an insulating substrate 101 made of Al2O3 or the like; a polycrystalline diamond layer 102 placed thereon; and a pair of interdigital electrodes 103, made of gold or the like, lying on the polycrystalline diamond layer 102.
When the ultraviolet sensor 100 is irradiated with light, electron-hole pairs are generated in the polycrystalline diamond layer 102. The interdigital electrodes 103 collect the electron and hole to output electric signals if a bias voltage is applied between the interdigital electrodes 103. Such a type of ultraviolet sensor, which is referred to as a photoconductor in general, is electrically insulative when it is not irradiated with light but is electrically conductive when it is irradiated with light.
In each known ultraviolet sensor described above, the polvcrvstalline diamond film functioning as a detector is exposed between the electrodes. Therefore, in order to measure short-wavelength light such as ultraviolet light with a wavelength 200 nm or less, the known ultraviolet sensors have the problems below.
The first problem is that organic compounds present in an atmosphere are decomposed during the irradiation because of the high energy of the short-wavelength light and the decomposed products adhere to the surfaces of the sensors. The adhesion of the decomposed products to the sensor surfaces reduces the amount of incident light to decrease the intensity of detected signals. The second problem is that moisture adsorbed on the sensor surfaces is dissociated into ions if the moisture is irradiated with strong ultraviolet light. Such ions cause a decrease in electrical resistance because they are caused to migrate on the sensor surfaces readily and slowly by an electric field applied between the electrodes. In this case, since the output of the sensors varies over several hundred seconds at the start or end of the ultraviolet irradiation, an output corresponding to the intensity of light cannot be obtained.
In order to solve these problems, an ultraviolet sensor disclosed in Japanese Unexamined Patent Application Publication No. 11-248531 is placed in a package, which is hermetically sealed and the atmosphere in which is replaced with an inert gas such as nitrogen, whereby reverse effects of the ambient atmosphere are omitted. However, the use of such a sealed package causes an increase in the number of manufacturing steps and an increase in the number of optical components and this causes an increase in manufacturing cost. Furthermore, since it is substantially impossible to completely remove moisture and organic compounds adsorbed on the inner surface of the package in advance, the moisture and the organic compounds remain semi-permanently in the package to cause contamination in the package and this causes a deterioration in properties in some cases. For the sensor, the sealed package has a window, made of ultraviolet-transmissive glass or sapphire, for introducing ultraviolet light into the package. Since the window absorbs ultraviolet light in some cases, obtained output is smaller than that of a sensor having no sealed structure depending on the wavelength of the ultraviolet light.
In general, a photodiode-type sensor containing a semiconductor material, such as silicon, other than diamond includes a depletion layer due to a Schottky or pn junction. This type of sensor has an advantage in that the depletion layer functioning as a detector is hardly affected by disturbance because it is placed in the solid diode, an advantage in that satisfactory properties can be obtained because a high electric field is applied only to the depletion layer, and other advantages. However, such a Schottky or pn junction cannot be substantially formed in any diamond film; hence, no photodiode-type sensor including any diamond film can be used.
The following sensor is known: a vertical ultraviolet sensor including a conductive substrate, a polycrystalline diamond layer placed thereon, and an electrode lying over the layer. FIG. 6 is a schematic sectional view showing the known vertical ultraviolet sensor. With reference to FIG. 6, the vertical ultraviolet sensor 104 includes a conductive substrate 105, made of silicon or the like, having low resistance; a polycrystalline diamond layer 102 placed thereon; and an electrode 106 which is made of gold or the like, which has a thickness of about 200 Å, and which lies over the polycrystalline diamond layer 102. In the vertical ultraviolet sensor 104, a bias voltage is applied between the conductive substrate 105 and the electrode 106.
The vertical ultraviolet sensor 104 is only slightly affected by organic compounds and moisture adsorbed thereon. However, there is a problem in that the vertical ultraviolet sensor 104 has unsatisfactory properties because the polycrystalline diamond layer 102 has a high density of grain boundaries present close to the conductive substrate 105 and therefore has a high density of crystal defects if the polycrystalline diamond layer 102 is formed by a vapor deposition process. The crystal defect density can be reduced by increasing the thickness of the polycrystalline diamond layer 102 and polishing the rear face thereof; however, this technique causes a serious increase in manufacturing cost and cannot therefore be applied to a sensor for mass production. Furthermore, there is a problem in that the vertical ultraviolet sensor 104 has low detection efficiency because the electrode 106 lies over the polycrystalline diamond layer 102 functioning as a detector and reflects or absorbs ultraviolet light.